This invention relates to control of an NOx accumulating catalytic converter using an NOx sensor and, more particularly, to the control of NOx catalyst regeneration by monitoring of the exhaust gas with an NOx sensor.
In gasoline engines which operate on lean fuel mixtures, the lean combustion conditions produce nitrogen oxides, called NOx, in the exhaust gas stream, which must be removed because of their damaging effects on the environment. NOx accumulating catalytic converters in which an NOx accumulator in the catalytic converter stores the NOx during lean-mixture operation and releases it again during rich-mixture operation of the engine are preferably used to remove nitrogen oxides from lean exhaust gas. During this process, the NOx is reduced in the catalytic converter by the reducing constituents contained in the rich mixture exhaust gas.
The quantity of NOx that the NOx accumulator in the storage catalytic converter can absorb is limited so that the NOx accumulator loses its absorption capability after extended lean-mixture operation of the engine. It is thus necessary to regenerate the NOx accumulator from time to time. This regeneration can be performed at predetermined fixed time intervals. However, this procedure has the disadvantage that the NOx accumulator is regenerated even if its NOx absorbing capacity is not yet exhausted, which leads to increased fuel consumption because of the increased frequency of operation of the engine in the rich-mixture state. It is also possible to calculate estimated loading of the NOx absorber from engine operation data but this produces only an approximate determination of the loading of the NOx accumulator. Consequently, a safety margin must be built in when using this regeneration method, likewise resulting in increased fuel consumption. It is further possible to directly determine the point in time when regeneration is needed by using an NOx sensor which detects a rise in the NOx signal that occurs when the NOx accumulator is full. However, in this case there remains the question of how long regeneration must be carried out under rich-mixture conditions in order to completely empty the NOx accumulator. Rich-mixture operation of the engine should, if possible, last no longer than is necessary to completely empty the NOx accumulator, since an unnecessary increase in fuel consumption will otherwise result.
German Offenlegungsschrift No. 195 11 548 discloses a method and arrangement for nitrogen oxide reduction in the exhaust gas of an internal combustion engine which is operated alternately in lean and in stoichiometric or enriched operation. During this process, the concentration of hydrocarbons, carbon monoxides and nitrogen oxides in the exhaust gas is measured downstream of an NOx accumulator in the direction of exhaust gas flow using a suitable sensor but precise knowledge of the regeneration interval is not possible with this method, so that fuel consumption is not optimal.
Accordingly, it is an object of the present invention to provide a method for controlling regeneration of an NOx accumulating catalytic converter which overcomes disadvantages of the prior art.
Another object of the invention is to provide an improved method for determining the regeneration interval of an NOx-accumulating catalytic converter in order to permit adjustment of the interval of rich-mixture operation of the engine to the necessary regeneration time of the NOx accumulator.
These and other objects of the invention are attained by providing an NOx sensor which also detects at least one exhaust-gas constituent that is an NOx reducing constituent to enable determination of the termination of NOx accumulator regeneration.
By using such a sensor for determining the regeneration interval of an NOx-absorbing catalytic converter of an engine that can run on a lean mixture according to the invention, both the beginning and the end of the regeneration interval can be determined because the sensor has, in addition to its NOx-detecting sensitivity, a cross-sensitivity to at least one exhaust-gas product that is a reducing constituent of the exhaust gas and that does not pass through the catalytic converter until the regeneration phase ends. Such a cross-sensitivity exists when the sensor, in addition to its designated sensitivity, can detect at least one other exhaust-gas constituent which does not appear or is not relevant during the non-regeneration phase, i.e., the NOx accumulation phase, and thus does not interfere with an NOx determination during the non-regeneration phase.
Preferably the reducing constituent of the exhaust gas to which the sensor is responsive is CO or NH3. Because both the start and the end of the regeneration phase can be precisely detected by this NOx sensor, a precise determination of the NOx accumulator regeneration interval is possible, which results in optimal fuel consumption in the lean running engine.
Such precise detection of the actual start and end of the regeneration interval for the accumulator opens up a large number of possibilities for engine management.
First, as already described, a sensor signal can be used to end the rich operation regeneration phase of the NOx accumulator and restore the engine to lean operating conditions.
Moreover, the sensor signal can be used to correct data in an NOx accumulator regeneration model stored in the vehicle""s engine management unit. It should be noted here that NOx accumulator regeneration, or its duration, is normally derived from an engine performance graph stored in the engine management unit. Since the stated performance graph only approximately represents the engine characteristics during the course of engine operation, the graph can be adjusted to the actual circumstances based upon the precisely determined regeneration interval in that, for example, from time to time regeneration is performed not on the basis of the stored performance graph, but rather using the measured sensor signals. Furthermore, the engine management unit can thus determine the extent of the deviation of the stored performance graph from reality. This can be done in accordance with a fixed procedure. For example, the duration of regeneration can be determined based on the sensor signals every fifth to five thousandth time, preferably every tenth to five hundredth time, or every one hundredth time, and/or following the engine warmup period regeneration can be performed in accordance with the actual sensor measurements every first to fifth time.
Furthermore, the sensor end signal, i.e., the point in time when regeneration stops, can be used to determine the state of ageing of the catalytic converter by comparing the actual duration of regeneration to the predetermined duration of regeneration stored in the engine management unit.